1. Field of the Invention
The invention relates to a diffuser-nozzle assembly for feeding air to an annular combustion chamber in a turbomachine such as an airplane turboprop or turbojet or a helicopter engine.
2. Description of the Related Art
In a conventional embodiment, a turbomachine has an annular combustion chamber that is fed with air by a high pressure compressor arranged upstream therefrom. The outlet from the compressor is connected via an axial or radial diffuser that is connected to a nozzle comprising substantially radial vanes at the inlet to the combustion chamber, which is itself mounted in an annular enclosure defined externally by an outer casing. Fuel is delivered via injectors to the annular end wall of the chamber, which injectors are regularly spaced apart and carried by the outer casing.
The performance of the combustion chamber depends to a very great extent on the performance of the injector system, which contributes to atomizing the fuel, i.e. to dispersing it in the form of fine droplets, and thereby serves to stabilize the combustion of the fuel by forming a recirculation zone. Fuel atomization is encouraged by a large head loss for the air in the injector system, and control over the recirculation zone is enhanced by feeding air uniformly to the injector system.
These criteria are difficult to satisfy when the outlet from the compressor comprises a diffuser-nozzle assembly of the axial type or of the centrifugal type, i.e. a radial diffuser and an annular nozzle.
At the outlet from the nozzle, it is observed that the Mach number downstream from the vanes is much lower substantially in line with the vanes than it is between the vanes. The flow of air at the outlet from the nozzle thus presents a Mach number with circumferential non-uniformities, thereby leading to an effective Mach number that differs from the Mach number corresponding to a flow that is uniform. This gives rise to large head losses when feeding the injector system and to distortions in the feed provided by the system, thereby running the risk of affecting the stability of combustion.
In order to minimize the impact of circumferential non-uniformities in the speed of air at the outlet from the nozzle, it is known to control the distance between the trailing edges of the vanes of the nozzle and the upstream end of the injector system as a function of the angle of orientation of the air stream.
Application FR 08/01063 in the name of the Applicant proposes a flared section downstream from the vanes of the nozzle in order to reduce the mean Mach number and reduce the circumferential non-uniformities in the Mach number of the air stream at the inlet to the combustion chamber.
Nevertheless, those known means do not enable a satisfactory solution to be provided to the problem of the Mach number being smaller in the wakes of the vanes of the nozzle.